1. Field of the Invention
The present invention pertains to a method of installing a combination drivepipe and casing string in an offshore well, using an improved drivepipe/casing assembly.
2. Background
Conventional practice in installing so-called drivepipe in offshore wells comprises driving a relatively large diameter string of pipe into the mud until refusal is encountered followed by conventional well drilling practices which are carried out through this so-called drivepipe or outer casing string by drilling successively smaller diameter holes and alternately setting casing strings until the desired hole depth is obtained. The drivepipe (also sometimes referred to as conductor or structural pipe) becomes the conduit for guiding the drillstring and serves as a conduit for fluids, such as drilling mud, which are returned to the surface during drilling operations.
The process of rigging up and rigging down for drilling, then setting casing, then drilling again is time consuming, expensive and subject to certain hazards. Accordingly, there has been a never-ending search for methods and apparatus to improve the well drilling and completion process, particularly with respect to the expensive, more hazardous and environmentally sensitive situations encountered in drilling and encasing offshore wells. The present invention is directed to an improved combination drivepipe and casing assembly as well as a unique method of driving, drilling and driving to final installation of a casing assembly and which overcomes several of the disadvantages of prior art practices.